It is a common practice, in the design of blown-film extrusion dies, when the film was of only one layer, to feed the polymer melt to the annular nozzle via a series of helical or spiral grooves. The helical grooves receive the melt from respective radial conduits, which convey the melt outwards from a central duct. The central duct lies on the axis of the die, i.e at the centre of the die.
This centre-fed-helical-grooves arrangement gave excellent results as far as evenness and quality of the film is concerned. (A key to economical production of plastic film is to ensure that the properties of the film are the same all the way around the circumference of the bubble of extruded material passing out through the annular nozzle.) The centre-fed-helical-groove arrangement is simple enough to engineer when the film is of only one layer. Two, three, four, etc, layers can also be accommodated with ease. In that case, the central ducts for conveying the separate materials for the layers are clustered around the axis of the die. Or, the several central ducts may be arranged annularly, i.e one inside another co-axially, around the axis.
But as the number of layers increases, it becomes more and more difficult for the designer to find room, on the axis of the die, for all the separate central ducts, and at the same time to ensure that all the spiral grooves of each of the layers receive an even amount of melt.
It is also a common practice among die designers, rather than arrange for the melt to be fed outwards from a central duct, then through the helical grooves and then to the annular nozzle, to arrange instead for the melt to be side-fed inwards into the annular nozzle from outside.
With an outside-fed, or side-fed, die, the problem of finding room for the melt-feed ducts on the inside of the die is eliminated, which is why outside-fed is preferred when the number of layers is high. But, with outside-fed, the designer has a more difficult task of ensuring that the melt is fed evenly all around the circumference of the die.
In the conventional outside-fed dies, the normal arrangement was for the melt to be conveyed around the die in a fanning-out configuration. That is to say, the designer arranged for the melt to be spread, from the melt-feed-port, directly right around the circumference.